A rotary engine, uses significantly fewer moving parts to create power than a reciprocating piston engine. Whereas a piston engine requires complex valve and camshaft mechanisms to control the timing of the intake and exhaust strokes, in the rotary alternative, the movement of the rotor itself opens and closes the intake and exhaust ports cut into the walls of the rotor housing.

Another significant fact is that the rod connecting the piston to the crankshaft moves linearly up and down, coming to a complete stop at TDC and BDC (Top/Bottom Dead Center) to reverse direction. In contrast, the rotor in a rotary engine continues to move in one direction, providing the smooth and low-vibration operation typical to these engines.

Thanks to its simple and low number of parts design, the rotary engine has the advantages of compact volume and low weight over the more common internal combustion engine. These advantages give rotary engine applications in various vehicles and devices, including automobiles, motorcycles, racing cars, aircraft, UAVs, and auxiliary power units.

As far as reliability, when compared to the classical piston engine, there are neither reciprocating masses nor cranks, valves, rods, or other failure-prone, complex parts. 2 rotors rotary engines contain only 3 moving parts (the 2 rotors with their internal hub gears, and the output shaft), which makes the engine more reliable, durable, and maintenance-friendly. Besides, these moving parts are in continuous unidirectional rotation, ensuring higher operating speeds, ease of balancing, and a low vibration level.

 

A summary of the prime advantages of the rotary engine is:

 

• A far higher power-to-weight ratio than a piston engine, when compared to a similar engine.

• More straightforward to package in small engine compartment spaces than an equivalent piston engine.

• No reciprocating parts

• Able to reach higher revolutions per minute than a piston engine

• Operating with almost no vibration

• Not prone to engine-knock

• Cost-effective to mass-production because the engine contains fewer parts

• Superior breathing, filling the combustion charge in 270 degrees of main-shaft rotation rather than 180 degrees in a piston engine

• Supplying torque for about two-thirds of the combustion cycle rather than one quarter for a piston engine

• A wider speed range gives greater adaptability.

• User-friendly to all kinds of fuels

• Easily adapted and highly suitable to use hydrogen fuel.

• It does not suffer from the "scale effect" to limit its size.

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The rotary design eliminates the internal reciprocating stresses by completely removing reciprocating internal parts and the absence of valves and valve trains, typically found in a piston engine.

The rotary engine is constructed with an iron rotor within a housing made of aluminum, which has a greater coefficient of thermal expansion. Thus, the engine is almost immune to catastrophic failure. A rotary engine that loses compression, cooling, or oil pressure, will lose a large amount of power and fail in a short time. It will, however, usually continue to produce some power during that time, allowing for a safer landing when used in an aircraft or UAV. Piston engines under the same circumstances are prone to seizing or breaking parts, which will almost certainly result in catastrophic failure of the engine and the instant loss of all power. This ensures that even a severely overheated rotary engine cannot seize, as is likely to occur in an overheated piston engine. This is a substantial safety benefit when used in aircraft or UAVs.

 

A summary of the prime disadvantages of the rotary engine is:

 

Thermal Efficiency - One of the rotary engine's major disadvantages is its low thermal efficiency. The long, thin, and moving combustion chamber results in slow and incomplete burning of the fuel mixture. Resulting in unburnt fuel entering the exhaust stream and the tendency of rotary engines to backfire. Fuel that is wasted, not being used to create power, leads to higher carbon emissions and lower fuel efficiency than piston engines. However, this drawback turns into an advantage by switching to Hydrogen fuel.

Acceleration and deceleration in average driving conditions also affect fuel economy. However, operating the engine at a constant speed and load, as in an APU, eliminates excess fuel consumption.

Rotary engines are also exceptionally well adapted to Alcohol in Gasoline mixes, such as E5 and E10, which are sold in Europe.

 

Seals -Another weakness of rotary engines comes from the rotor sides and apex sealing. Imperfect sealing between the edges of the rotor and the housing – for example, due to wear, insufficient centrifugal force on the lower RPM ranges, or the fact that combustion occurs only in one section of the rotary engine, resulting in a high-temperature difference between two adjacent chambers. Consequently, the different expansion coefficients of the materials lead to suboptimal rotor sealing and can result in combustion gas leaking into the next chamber. However, this is not a critical problem since leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the main shaft compartment, as in piston engines. And all in all, sealing has significantly improved over the years.

 

Oil consumption - By design, the rotary engine burns oil. Oil is directly injected into the combustion chamber to keep the rotor seals adequately lubricated, but it also means more bad stuff that comes out of the tailpipe.

 

Rotary engines in airborne usage

In principle, rotary engines are ideal for light aircraft and UAV usage, being lightweight, compact, almost vibrationless, and with a high power-to-weight ratio. Further aviation benefits of these engines include:

 

Rotors cannot seize since rotor housing expands greater than rotors;

The engine is less prone to the severe condition known as "engine-knock", which can destroy an airborne piston engine in mid-flight.

The engine is not susceptible to "shock-cooling" during descent;

The engine does not require an enriched mixture for cooling at high power;

with no reciprocating parts, there is less vulnerability to damage when the engine revolves at a higher rate than the designed maximum. The limit to the revolutions is the strength of the main bearings.

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Rotary engines as range extenders

Due to its compact size and the high power-to-weight ratio of the rotary engine, it has been proposed for electric vehicles as range extenders to provide extra power when the electric battery levels are low. There have been several cars incorporating a series hybrid powertrain arrangement.

A rotary engine used only as a generator has packaging, noise, vibration, and weight distribution advantages when used in a vehicle, maximizing interior passenger and luggage space. The engine/generator may be at one end of the vehicle with the electric driving motors at the other, connected only by cables. 

Mitsueo Hitomi, the global powertrain head of Mazda, stated,

"A rotary engine is ideal as a range extender because it is compact and powerful while generating low vibration".